1. Field of the Invention
The present invention relates to a technique for transmitting/receiving data in units of channels by using a two-dimensional spread code with which spreading is made in a time direction and a frequency direction.
2. Description of the Related Art
In recent years, in a mobile communication field, attention has been focused on, for example, an OFDM-CDMA method, into which an OFDM (Orthogonal Frequency Division Multiplexing) modulation method and a CDMA (Code Division Multiple Access) method are combined, as a multi-carrier modulation method. The OFDM modulation method is a modulation method having a high frequency use efficiency utilizing a plurality of mutually orthogonal subcarriers, whereas the CDMA method is a modulation method using a spread spectrum communication method the interference immunity of which is high. The OFDM-CDMA method, into which these two methods are combined, spreads at least either of time and frequency directions by using a two-dimensional spread code with which spreading can be made in the time and the frequency directions. This method is recited, for example, in Patent Documents 1 and 2.
In a mobile communication, the state of a propagation path of a channel varies by situation. If the state of a propagation path becomes worse, a transmission characteristic or a system capacity degrades. With the conventional technique recited in Patent Document 1, a transmitting side sets spreading factors in time and frequency directions according to the state of a propagation path of a channel in order to prevent a transmission characteristic and a system capacity from degrading. For example, the spreading factor in the frequency direction is set to a smaller value as a maximum delay time in the propagation path becomes longer, so that orthogonality between spread codes is prevented from degrading. Additionally, the spreading factor in the time direction is set to a smaller value as a maximum Doppler frequency in the propagation path becomes higher, so that orthogonality between spread codes is prevented from degrading.
With the conventional technique recited in Patent Document 1, a spreading factor that differs by user (receiving side) is applied according to the state of a propagation path, or the same spreading factor is applied to a plurality of users, when a transmitting side transmits a pilot symbol or the like for estimating a channel.
If a different spreading factor is applied, spread codes the number of which is equal to that of users are required. As a result, the capacity of the entire system becomes small, leading to a decrease in the efficiency of using the system capacity. Additionally, since power equivalent to the number of users is required, also power consumption becomes higher.
In the meantime, if the same spreading factor is applied, power consumption can be further reduced. This is because each user makes despreading with the same spreading factor. However, since the reception characteristic of each user differs depending on the state of each propagation path, it is very difficult to directly identify an optimum spreading factor for each user. Therefore, it is vital to enable a plurality of users to receive data with a spreading factor according to the state of each propagation path while keeping the efficiency of using the system capacity high.    Patent Document 1 : Japanese Published Unexamined Patent Application No. 2003-46474    Patent Document 2 : Japanese Published Unexamined Patent Application No. 2004-48117